Shell Nanocable−Carbon Microfiber

LETTER pubs.acs.org/NanoLett

Flexible Zn2SnO4/MnO2 Core/Shell Nanocable-Carbon Microfiber Hybrid Composites for High-Performance Supercapacitor Electrodes Lihong Bao, Jianfeng Zang, and Xiaodong Li* Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208, United States

bS Supporting Information ABSTRACT: We demonstrate the design and fabrication of a novel flexible nanoarchitecture by facile coating ultrathin (several nanometers thick) films of MnO2 to highly electrical conductive Zn2SnO4 (ZTO) nanowires grown radially on carbon microfibers (CMFs) to achieve high specific capacitance, high-energy density, high-power density, and long-term life for supercapacitor electrode applications. The crystalline ZTO nanowires grown on CMFs were uniquely served as highly conductive cores to support a highly electrolytic accessible surface area of redox active MnO2 shells and also provide reliable electrical connections to the MnO2 shells. The maximum specific capacitances of 621.6 F/g (based on pristine MnO2) by cyclic voltammetry (CV) at a scan rate of 2 mV/s and 642.4 F/g by chronopotentiometry at a current density of 1 A/g were achieved in 1 M Na2SO4 aqueous solution. The hybrid MnO2/ZTO/CMF hybrid composite also exhibited excellent rate capability with specific energy of 36.8 Wh/kg and specific power of 32 kW/kg at current density of 40 A/g, respectively, and good long-term cycling stability (only 1.2% loss of its initial specific capacitance after 1000 cycles). These results suggest that such MnO2/ ZTO/CF hybrid composite architecture is very promising for next generation high-performance supercapacitors. KEYWORDS: Supercapacitor, MnO2, Zn2SnO4 nanowire, carbon microfiber, flexible, composite

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o meet urgent needs for sustainable and renewable power sources in modern electronic industry, many efforts have been made in developing flexible, lightweight and environmentally friendly energy storage devices, such as supercapacitors1-3 and batteries.4,5 Supercapacitors, also called ultracapacitors, electrochemical capacitors (ECs),6 or electrical double layer capacitors (EDLCs) have become some of the most promising candidates for next-generation power devices because of their high power density, fast charging/discharging rate, sustainable cycling life (millions of cycles), and excellent cycle stability.1-3,7,8 Carboneous materials, such as carbon nanotube networks,9-11 graphene nanosheets12-16 and conducting polymers,17-20 transition-metal oxides,21-24 and hybrid composites25-33 have been used to fabricate flexible supercapacitor electrodes. Among these candidated electrode materials, MnO2 exhibits many intriguing characteristics, such as low cost, environmental friendliness, and natural abundance, suggesting it as the most promising electrode material for next generation supercapacitors. The theoretical specific capacitance of MnO2 is 1370 F/g.34 However, due to its poor electric conductivity (10-5-10-6 S/cm) such high theoretical capacitance has not been achieved in experiments. Some high-performance results have been reported only from MnO2 nanometer-thick thin films and/or nanosized particles. In addition, when the loading of MnO2 is in a high weight percentage in the electrode, the MnO2 is densely packed and then has only very limited accessible surface area for participating electrochemical r 2011 American Chemical Society

charge storage process, which remarkably increases the contact resistance and in turn decreases the specific capacitance.27 Therfore, to maximize utilization of the pseudocapacity of MnO2, keeping its thin film morphology while providing reliable electrical connection becomes one of the essential criteria in designing high-performance electrodes for MnO2-based electrochemical supercapacitors. One promising approach to realizing the practical application of MnO2 and improving its electrical conductivity is to incorporate MnO2 nanostructures or nanometer-thick thin films into carbon-based materials,26,27,31,35-40 such as carbon nanotube networks,26,27,37,39 graphene sheets,29,36 and conductive polymers.31,38-40 Recently, Yan et al. demonstrated an improved electrochemical capacitive behavior by coating MnO2 onto SnO2 nanowires grown on stainless steel substrate,41 nevertheless, the inflexible/rigid nature of stainless steel substrate prevents them from practical applications in harsh environments such as folding/twisting conditions. Here, we demonstrate the design and fabrication of a novel hybrid nanoarchitecture by facile coating ultrathin (several nanometers thick) MnO2 film to highly electrical conductive Zn2SnO4 (ZTO) nanowires (conductivity: 102103 S/cm) grown on flexible carbon microfibers (CMFs) to Received: December 2, 2010 Revised: January 30, 2011 Published: February 09, 2011 1215

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Nano Letters achieve high specific capacitance, high-energy density, highpower density, and long-term life for supercapacitor electrode applications. The crystalline ZTO nanowires grown radially on CMFs were served uniquely as highly electrical conductive cores to support redox active MnO2 shells with highly electrolytic accessible surface area and to provide reliable electrical connections to the MnO2 shells, enabling full utilization of MnO2 and fast electric and ionic conduction through the electrode. The maximum specific capacitances of 621.6 F/g (based on pristine MnO2) by cyclic voltammetry (CV) at a scan rate of 2 mV/s and 642.4 F/g by chronopotentiometry at a current density of 1 A/g were achieved in 1 M Na2SO4 aqueous solution. The hybrid MnO2/ZTO/CMF composite also exhibited excellent rate capability with the specific energy of 36.8 Wh/kg and the specific power of 32 kW/kg at the current density of 40 A/g as well as good long-term cycling stability (only 1.2% loss of its initial specific capacitance after 1000 cycles), which were derived from galvanostatic (GV) charge-discharge measurements. These results suggest that such MnO2/ZTO/CMF hybrid composite hierarchical architecture is very promising for next generation high-performance supercapacitors. High-density ZTO nanowires were fabricated on commercial woven CMFs with a diameter of several micrometers by a simple vapor transport method in a horizontal tube furnace (for details, see Supporting Information). To faciliate the electrochemical tests, one bundle of ZTO nanowire/CMF were taken out from the woven structures and then immersed into a mixed solution containing 0.1 M Na2SO4 (Sigma-Aldrich) and 0.1 M KMnO4 (Sigma-Aldrich). In addition, to enhance the mechanical stability and electrical conductivity of the electrode after coating with MnO2, the MnO2/ZTO/CMF hybrid composite was heat treated at 120 °C for 12 h in air (for details, see Supporting Information). Figure 1a-d shows the morphology and microstructure of ZTO nanowires on the woven CMFs. It is revealed that high-density of ZTO nanowires were radially grown on the CMFs. The ZTO nanowires covered all around the CMFs, even the space between the CMFs (for details, see Supporting Information). These ZTO nanowires have lengths of tens of micrometers and diameters of around 80 nm. To investigate the microstructure of ZTO/MnO2 core/shell nanocables, TEM imaging and selected-area electron diffraciton (SAED) analysis were employed. A typical TEM image of an individual nanocable is shown in Figure 1e. The SAED pattern (Figure 1f) can be indexed as single crystalline inverse spinel structure of Zn2SnO4 with [110] as zone axis (JCPDS No. 24-1470, Fd3m, a = b = c = 0.865 nm). No MnO2-related diffraction pattern or spot can be found, indicating that the coated MnO2 is amorphous. A rough and amorphous MnO2 shell with a thickness of several nanometers was found on the ZTO nanowire surface, as shown in Figure 1g,h. The elemental spatial distributions across the ZTO/MnO2 core-shell nanocable were characterized by energy-dispersive spectroscopy (EDS) in the form of line scan profiles of individual elements Zn, Sn, O and Mn, as shown in Figure 1j. The peaks of the Zn and Sn line-scan profiles are located in the center of the profile of Mn, confirming the coreshell configuration of the ZTO/MnO2 nanocable. The oxidation state of Mn atoms in the as-coated MnO2 shells on ZTO nanowire cores was determined as ∼4.0 by X-ray photoelectron spectroscopy (XPS) (for details, see Supporting Information). The amorphous nature of the MnO2 coating is more favorable for supercapacitor applications compared with the previously reported crystalline MnO2 coatings.42 This core-shell ZTO/

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MnO2 architecture has a potential to improve the electrochemical performance of the MnO2/ZTO/CMF hybrid composite. The thin layer of MnO2 enables the fast and reversible faradic reaction by shortening the ion diffusion path and low weight loading of MnO2 can achieve high specific capacitance. Furthermore, the high specific area of core ZTO nanowires provides highly conductive channels to effectively transport electrolyte. By using this nano/micro hierarchical design, we can fully utilize the outstanding electrochemical performance of MnO2, realizing high-energy density and high-power density characteristics for real electrochemical capacitor applications. Figure 2a shows the CV curves of the MnO2/ZTO/CMF hybrid composite electrode at scan rates of 2, 5, 10, 20, 50, 100 mV/s with potential windows ranging from 0 to 0.8 V versus Ag/ AgCl in 1 M Na2SO4 aqueous solution. The shapes of these curves are quasi-rectangular, indicating the ideal electrical double-layer capacitance behavior and fast charging/discharging process characteristic. The MnO2-coated ZTO nanowires involved redox reactions in the cyclic voltammetry tests as the Mn atoms in the overlayer were converted into higher/lower valence states, which were induced by intercalation/extraction of protons (H3Oþ) or alkali cations (Naþ) into/out of the ZTO/MnO2 core/shell nanocables and could be expressed as34 MnO2 þ Mþ þ e- T MnOOM, Mþ ¼ Naþ or H3 Oþ

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To demonstrate the electrochemical performance benefits of the MnO2/ZTO/CMF hybrid composite, CV tests were performed on the respective MnO2/CMF and ZTO/CMF composites, as shown in Figure 2b,c. For the MnO2/CMF composite (Figure 2b), the CV curves obtained at different scan rates also show quasi-rectangular shapes, however, the separation between leveled anodic and cathodic currents at the same scan rates is much smaller than the MnO2/ZTO/CMF composite, indicating smaller specific capacitances. For the ZTO/CMF composite (Figure 2c), the CV curves do not show the normal regular shape and the leveled current separation between leveled anodic and cathodic currents is much smaller, suggesting the poor electrochemical performance of the ZTO/CMF composite. The specific capacitances calculated from the CV curves (for details, see Supporting Information) with different scan rates are shown in Figure 2d. At the scan rate of 2 mV/s, the specific capacitance of the MnO2/ZTO/CMF hybrid composite can achieve 621.6 F/g (based on the mass of pristine MnO2), while those of the MnO2/CMF and ZTO/CMF composites are only 46.6 and 5.6 F/g, respectively. The CV tests performed on carbon microfiber electrode indicated that only 0.15 F/g of specific capacitance can be obtained at the scan rate of 2 mV/s (for details, see Supporting Information). Therefore, in the MnO2/ZTO/CMF hybrid composite the capacitances contributed from ZTO nanowires and CMFs are negligible. The CV tests suggest that compared with the MnO2/CMF composite, through growth of ZTO nanowires on CMFs as template to coat MnO2, the electrochemical accessible surface area is remarkably increased due to the large surface to volume ratio of the ZTO nanowires, resulting in the significant improvement of the specific capacitance. Additionally, the high specific capacitance value confirms that such design and fabrication of the MnO2/ ZTO/CMF hybrid composite allows maximizing the utilization of the electrochemical performance of MnO2. In addition to the flexible nature of CMFs, CMFs can provide more high surface 1216

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Figure 1. Structural characterization of MnO2/ZTO/CMF hybrid composite. (a-d) SEM images of ZTO nanowires grown radially on the woven CMFs. (e) TEM image of MnO2-coated ZTO nanowire and (f) corresponding SAED pattern. (g) HRTEM image of an individual ZTO/MnO2 nanocable, showing that the rough and thin amorphous layer on the surface of the nanowire is amorphous MnO2 shell, inset is the corresponding FFT pattern. (h) Lattice-resolved HRTEM image of the ZTO/MnO2 core/shell nanocable, showing the detailed interface of crystalline ZTO core and amorphous MnO2 shell. (i,j) TEM image and line-scan profiles (indicated by a line in panel i) of the ZTO/MnO2 core/shell nanocable, showing Zn, Sn, O, and Mn elemental profiles. (k) A representative EDS spectrum taken on the nanocable. Scale bars in (a-d,e,g,h,i) are 1 mm, 50 μm, 100 μm, 20 μm, 200 nm, 10 nm, 5 nm, and 200 nm, respectively.

Figure 2. (a-c) Cyclic voltammetry curves of the MnO2/ZTO/CMF, MnO2/CMF, and ZTO/CMF composites at different scan rates in 1 M Na2SO4 aqueous solution, respectively, showing the high-electrochemical performance of the MnO2/ZTO/CMF hybrid composite compared with that of MnO2/CMF and ZTO/CMF composites. (d) Specific capacitances of MnO2/ZTO/CMF (blue), MnO2/CMF (red), and ZTO/CMF (black) composites at different scan rates derived from cyclic voltammetry. 1217

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Figure 3. Galvanostatic (GV) constant-current charge/discharge performance of MnO2/ZTO/CMF hybrid composite electrode. (a) Constantcurrent charge/discharge curves of the MnO2/ZTO/CMF hybrid composite electrode at different current densities. (b) Specific capacitances of of the MnO2/ZTO/CMF hybrid composite at different current densities. (c) Ragone plot of the estimated specific energy and specific power at various charge/discharge rates (current densities). (d) Charge/discharge cycling test at the current density of 10 A/g, showing 1.2% loss after 1000 cycles; inset shows the galvanostatic charge/discharge cyclic curves of the first and last 10 cycles.

area to support more ZTO nanowires per unit substrate area. The electrochemical performance benefits of using CMFs as the substrate were demonstrated with reference to the flat conductive stainless steel (SS) substrate. The MnO2/ZTO/CMF hybrid composite has a better electrochemical performance than the MnO2/ZTO/SS composite (for details, see Supporting Information). Rate capabilitiy is one of the important factors for evaluating the power applications of supercapacitors. The constant-current galvanostatic (GV) charge/discharge curves of the asprepared MnO2/ZTO/CMF hybrid composite at different current densities are shown in Figure 3a. The charging/discharging cycling curves have a very symmetric nature, indicating again that the composite has a good electrochemical capacitive characteristic and superior reversible redox reaction. This symmetric nature of the charging/discharging cycling curves can be maintained even at a low density of 1 A/g, as shown in Figure 3a. The specfic capacitances derived from the discharging curves (for details, see Supporting Information) at different charge/discharge rates (current densities) are shown in Figure 3b. The specfic capacitance of the composite at the current density of 1 A/g was calculated to be 642.3 F/g based on the mass of the pristine MnO2, which is comparable with the results of the CV tests. Although the specific capacitance is lower than the previously reported 800 F/g,41 our prepared MnO2/ZTO/CMF hybrid composite electrode is highly flexible and can be applied even in a harsh environment such as folding/twisting conditions. To study the flexibility of the composite electrodes, we investigated the electrochemical performance of the MnO2/ZTO/ CMF hybrid composite electrodes under folding/twisting conditions; no apparent changes were observed in the electrochemical tests (for details, see Supporting Information). This confirms the highly flexible nature of the MnO2/ZTO/CMF

hybrid composite electrodes for supercapacitors. At a very high current density of 40 A/g, the specific capacitance remained at 413.9 F/g. Such superior rate capability in the MnO2/ZTO/ CMF hybrid composite can be attributed to the reduced diffusion path of ions, highly accessible surface area and increased electrical conductivity by utilizing ZTO nanowires radially grown on carbon fibers as supporting backbones to coat MnO2. Specific energy and specific power are the two key factors for evaluating the power applications of electrochemical supercapacitors. A good electrochemical supercapacitor is expected to provide high energy density or high specific capacitance at high charging-discharging rates (current densities). Figure 3c shows the Ragone plot for the MnO2/ZTO/CMF composite electrode at the potential window of 0.8 V in 1 M Na2SO4 aqueous solution. The specific energy decreases from 57.1 to 36.8 Wh/ kg, while the specific power increases from 0.8 to 32 kW/kg as the galvanostatic (GV) charge/discharge current increased from 1 to 40 A/g. These values are much higher than those of conventional supercapacitors in Ragone plot.1 Most importantly, the highest specific power value, 32 kW/kg, can meet the power demands of the PNGV (Partnership for a New Generation of Vehicles),43-45 demonstrating the capability of MnO2/ZTO/CMF hybrid composite electrodes for electrochemical supercapacitors as power supply components in hybrid vehicle systems. Another important requirement for supercapacitor applications is cycling capability or cycling life. The cycling life tests over 1000 cycles for the MnO2/ZTO/CMF hybrid composites at a current density of 10 A/g were carried out using constantcurrent galvanostatic (GV) charge/discharge cycling techniques in the potential windows ranging from 0 to 0.8 V. Figure 3d shows the specific capacitance retention of the MnO2/ZTO/ CMF hybrid composite as a function of charge/discharge cycling numbers. The composite electrode showed only 1.2% loss in the 1218

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supporting backbones for coating amorphous MnO2 to effectively transport electrolytes and shorten the ion diffusion path. These characteristics offer the excellent electrochemical performance of the MnO2/ZTO/CMF hybrid composites, such as high specific capacitance, good charge-discharge stability, excellent rate capability and long-term cycling life, high specific energy, and high specfic power. These results suggest that such MnO2/ZTO/CMF composite architecture is very promising for next generation high-performance supercapacitors.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details for the device preparation, characterization, and additional supporting data. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Figure 4. (a) Nyquist plot of the EIS of the MnO2/ZTO/CMF (black squares), MnO2/CMF (red circles), and ZTO/CMF (blue triangles) composites. (b) The equivalent circuit diagram of different elements from the EIS analysis.

specific capacitance after 1000 charge-discharge cycles and the last 10 cycles remained almost the same shape of chargedischarge curves with the first 10 cycles (insets in Figure 3d), illustrating the excellent long-term cyclability of the composite electrode. The enhanced electrochemical performance of the hybrid composite electrode using ZTO nanowires radially grown on CMFs as the template to coat MnO2 shell was further confirmd by the electrochemical impedance spectroscopy (EIS) measurements in the same setup as the CV and GV tests. Figure 4a shows the Nyquist plots of the EIS spectra of ZTO/CMF (blue triangles), MnO2/CMF (red circles), and MnO2/ZTO/CMF (black squares) composites, respectively. The EIS data can be fitted by a equivalent circuit consisting of a bulk solution resistance Rs, a charge-transfer Rct, a pseudocapacitive element Cp from redox process of MnO2, and a constant phase element (CPE) to account for the double-layer capacitance, as shown in Figure 4b. The bulk solution resistance Rs and charge-transfer resistance Rct can be obtained from the Nyquist plots, where the high frequency semicircle intercepts the real axis at Rs and (Rs þ Rct), respectively.46,47 The solution resistance Rs of these three composites was measured to be 8.7, 5.5, and 10.6 Ω, respectively, while the charge-transfer resistance Rct was calculated to be 1.6, 13.4, and 4.9 Ω, respectively. This clearly demonstrates the reduced charge-transfer resistance of the MnO2/ZTO/CMF hybrid composite electrode by using ZTO nanowires radially grown on CMFs as the template to coat MnO2 thin layer compared with that of using CMFs alone to coat MnO2 directly. In addition, the charge-transfer resistance Rct, also called Faraday resitance, is a limiting factor for the specific power of the supercapacitor.2,48 It is the low Faraday resistance that results in the high specific power of the MnO2/ZTO/CMF hybrid composite electrode. In summary, a simple and cost-effective approach is developed to fabricate flexible supercapacitors based on MnO2/ZTO/CMF hybrid composite electrodes. In such a composite, the thin amorphous MnO2 layer enables fast and reversible redox reaction to improve the specific capacitance, while the ZTO nanowires grown radially on CMFs provide highly conductive

Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the U.S. Army Research Office under Agreement/Grant W911NF-07-1-0320 and the National Science Foundation (CMMI-0653651 and CMMI-0968843). The authors thank Dr. Douglas Blom at the University of South Carolina EM Center for TEM technical support. ’ REFERENCES (1) Simon, P.; Gogotsi, Y. Materials for electrochemical capacitors. Nat. Mater. 2008, 7 (11), 845–854. (2) Conway, B. E. Electrochemical supercapacitors: Scientific fundamentals and technological applications; Kluwer Academic/Plenum Publishers: New York, 1997. (3) Miller, J. R.; Simon, P. Materials science - Electrochemical capacitors for energy management. Science 2008, 321 (5889), 651–652. (4) Nishide, H.; Oyaizu, K. Materials science - Toward flexible batteries. Science 2008, 319 (5864), 737–738. (5) Armand, M.; Tarascon, J. M. Building better batteries. Nature 2008, 451 (7179), 652–657. (6) Winter, M.; Brodd, R. J. What are batteries, fuel cells, and supercapacitors?. Chem. Rev. 2004, 104 (10), 4245–4269. (7) Kotz, R.; Carlen, M. Principles and applications of electrochemical capacitors. Electrochim. Acta 2000, 45 (15-16), 2483–2498. (8) Burke, A. R&D considerations for the performance and application of electrochemical capacitors. Electrochim. Acta 2007, 53 (3), 1083– 1091. (9) Hu, L. B.; Choi, J. W.; Yang, Y.; Jeong, S.; La Mantia, F.; Cui, L. F.; Cui, Y. Highly conductive paper for energy-storage devices. Proc. Natl. Acad. Sci. U.S.A. 2009, 106 (51), 21490–21494. (10) Kaempgen, M.; Chan, C. K.; Ma, J.; Cui, Y.; Gruner, G. Printable thin film supercapacitors using single-walled carbon nanotubes. Nano Lett. 2009, 9 (5), 1872–1876. (11) Hu, L. B.; Pasta, M.; La Mantia, F.; Cui, L. F.; Jeong, S.; Deshazer, H. D.; Choi, J. W.; Han, S. M.; Cui, Y. Stretchable, porous, and conductive energy textiles. Nano Lett. 2010, 10 (2), 708–714. (12) Zhang, L. L.; Zhou, R.; Zhao, X. S. Graphene-based materials as supercapacitor electrodes. J. Mater. Chem. 2010, 20 (29), 5983–5992. (13) Wang, Y.; Shi, Z. Q.; Huang, Y.; Ma, Y. F.; Wang, C. Y.; Chen, M. M.; Chen, Y. S. Supercapacitor devices based on graphene materials. J. Phys. Chem. C 2009, 113 (30), 13103–13107. 1219

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